The compound obtained by the present invention, which is represented by the formula (2):
[chemical name: threo-3-(3,4-dihydroxyphenyl)-L-serine (common name: Droxidopa)], is a drug that has been known to be useful as an agent for treatment of peripheral orthostatic hypotension (see for example, Patent Literature-1) or an agent for treatment of Parkinson's disease (see for example, Patent Literature-2).
As examples of the process for preparing Droxidopa, there has been known a process by reacting a compound represented by the formula (1):
[chemical name: threo-N-phthaloyl-3-(3,4-dihydroxyphenyl)-L-serine (hereinafter, referred to as “Compound (1)”)] with hydrazine (see for example, Patent Literature-3) and a process by removing a protecting group of threo-N-benzyloxycarbonyl-3-(3,4-dibenzyloxyphenyl)-L-serine represented by the formula (3):
using a palladium-carbon and the like as a catalyst under a hydrogen atmosphere (see for example, Patent Literature-4) and the like. However, in the case of the former process, there are some problems of requiring a large quantity of reaction solvents because phthalhydrazide as by-product is crystallized and precipitated out, and further requiring some troublesome operations for removing this by-product and additional amounts of the solvents, and the others, and in the case of the latter process, there are some problems of an operation for removing the catalyst used being troublesome, in addition to a process for preparing the above-mentioned compound represented by the formula (3) being troublesome, and these conventional processes thus cannot necessarily be sufficient industrially.
In general, it is known that amines can be used at converting a phthalimido group into an amino group, but there has never been a disclosure that the compound of the formula (1) is reacted with methylamine to obtain the above-mentioned threo-3-(3,4-dihydroxyphenyl)-L-serine represented by the formula (2).